1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser (VCSEL) diode and a method for manufacturing the same, and in particular to a vertical cavity surface emitting laser diode which can control a polarization mode stably and a method for manufacturing the same.
2. Related Art
A semiconductor light emitting device such as a semiconductor laser or a vertical cavity surface emitting laser (VCSEL) is widely used as a light source for such an optical disc system as a CD (a compact disc) or a DVD (a digital versatile disc), or a barcode reader, including an optical communication field. When the semiconductor light emitting device is used in various application fields including these optical communications, it is important to obtain single-mode operation for three modes of “longitudinal mode”, “transverse mode” and “polarization mode” present in a semiconductor laser. Nowadays, a semiconductor laser diode with an edge emission type mainly used in the optical communication field operates stably in the polarization mode. This is because a resonant cavity in the edge emission laser diode is constituted of a waveguide, a reflectance on an end face of the waveguide is larger in linear TM (Transverse Magnetic) polarization than in TE (Transverse Electric) polarization, and electric field vector oscillates due to TE wave in a direction parallel to a semiconductor substrate. Single-mode operation for the longitudinal mode can also be realized by a distributed feedback (DFB) structure and one for the transverse mode can be achieved with a narrow stripe structure.
On the other hand, in the surface emitting laser such as VCSEL, since a resonant cavity is extremely short, the longitudinal mode operates in a single mode, and the transverse mode can operate in a single mode with a narrow current confinement structure obtained by selective oxidation of a layer with aluminum (Al) high concentration or proton implantation.
As to the polarization mode, VCSEL inherently lacks strong polarization anisotropy due to the symmetry of the devices. The linear TE polarization is free to be randomly oriented in the plane of the active region since there is no gain difference between two orthogonal polarized waves in the active layer itself of VCSEL devices. Polarization instability of VCSEL results from difficulty in application of such means that a reflectance of a reflecting mirror for the polarized waves in a specific orientation or direction is increased due to no gain difference between orthogonal polarized waves. For this reason, switching between polarized modes of VCSEL occurs easily due to small fluctuation in external conditions such as a temperature or a driving current, which largely influences optical magnetic recording or coherent optical communication system which directly utilizes polarization states of laser beam. Even when ordinary data communication is performed, polarization instability causes excessive noises or modal competition such a problem as increase in error or restriction to a transmission band arises. Therefore, control (stabilization) for the polarization mode is one of important problems to be solved in order to achieve an actual application of the VCSEL.
Since the importance of polarization control was indicated, conventional approaches for solving the problem about the polarization control have been proposed as follows:    (1) Structure where metal dielectric diffraction grating is assembled in a Distributed Bragg Reflector (DBR) mirror constituted of a semiconductor multi-layered.    (2) Structure where asymmetry has been taken in a mesa shape in a device    (3) Structure where production is made on an off angle substrate
The approach (1) of the three approaches is a method where metal fine wires are arranged on a DBR mirror in a specific orientation so that a reflectance of the DBR mirror to a polarized light in a specific orientation is made high. Since the reflectance of the mirror to a polarized light parallel to the metal wires becomes high, the method is effective to a certain extent for the polarization stability, but it is difficult to manufacture a device having such a structure because it is necessary to form each metal wire with a width equal to or less than light wavelength of laser.
The approach (2) of taking asymmetry in a mesa shape of a device is disclosed in Japanese patent Application Laid-open No. 11-54838 (herein, also called Patent Literature 1), for example. As shown in FIGS. 21A and 21B, stress is unequally (anisotropically) applied to an active layer at the center of a mesa M by forming stress adding regions 24 around the mesa M, so that stress is generated anisotropically. Incidentally, FIGS. 21C and 21D are diagrams of a stress distribution corresponding to the structure shown in FIGS. 21A and 21B, respectively. A gain difference between orthogonal polarized waves occurs due to such stress, and only polarized waves in a specific direction become preferential so that polarization controllability can be increased.
Similarly, as shown in FIG. 22, IEEE Photon. Technol. Lett., Vol. 14, No. 8, 1034 (2002) (hereinafter, also called Non-Patent Literature 1) also discloses that T-shaped projection shapes 24 are respectively added to both side of a cylindrical mesa M. With such a structure, the layers with Al high concentration (Al0.9Ga0.1As layer) of the T-shaped fine wire portion of a DBR mirror are all oxidized by a selective oxidization process, and anisotropic strain is applied on an active layer positioned at the center of the mesa M by strong stress occurring due to volume shrinkage, which results in increase in polarization controllability.
IEEE Photon. Technol. Lett., Vol. 6, No. 1, 40 (1994) (hereinafter, also called Non-Patent Literature 2) describes that current injection to an active layer is made symmetric to achieve polarization control by employing a dumbbell-shaped mesa structure as shown within a circle 25 in FIG. 23. The stress adding region 24 or the asymmetrical mesa structure 25 causes such a problem as insufficiency in device productivity or reproduction easiness due to complication in device working step, or insufficient polarization controllability like the above-described approach (1).
On the other hand, the approach (3) using an off-angle substrate utilizes such a fact that an active layer is formed on a crystal plane with a big off-angled orientation such as (311) A plane or (311) B plane in order to increase a gain to polarization state in a certain orientation and the gain depends on a crystal orientation. In this approach, a strong extinction ratio between orthogonal polarized waves is obtained and controllability in a polarization mode is excellent. However, it is difficult to achieve high quality crystal for the growth on off-angle substrate, as compared with the case of a non-off-angle (100)-oriented substrate, which causes such a problem that it becomes difficult to obtain a high power output. Further, in a oxidized-confined VCSEL diode with an off-angle substrate, the shape of oxide-aperture to confine the current is distorted according to a difference in oxidation rate (anisotropic oxidation) due to a crystal orientation, which results in difficulty in control on a beam shape.
The VCSEL diode includes the problem about the polarization mode control, but it has various advantages including a low threshold operation, a lower power consumption, a high slope efficiency, possibility of fast modulation, small beam divergences and easiness of coupling with an optical fiber, excellent mass productivity due to no-need for an end face cleavage, as compared with the edge emission type semiconductor laser. Further, since it is possible to integrate many laser devices on a substrate in a 2-dimensional array, much attention has been paid to the VCSEL device as a key device in an optical electronics field including a fast optical LAN (Local Area Network), an optical interconnect or the like. Accordingly, it is strongly desired to provide a surface emitting type semiconductor laser device such as VCSEL which solves the above-described problems and has an improved polarization controllability and an excellent mass productivity.
As described above, in the VCSEL device fabricated on a non-off-angle substrate with a (100)-oriented plane or the like, since a gain difference between orthogonal polarized waves does not occur in an active layer due to symmetry of a crystal structure, switching between polarization states is caused easily, which results in such a problem that it is difficult to control the polarization mode.